Optical fiber for wavelength division multiplexing

ABSTRACT

A fiber suitable for use in high capacity optical fiber network operative with wavelength division multiplexing. Contemplated systems can utilize span distances in excess of 100 km, signal amplification within spans, and provide plural multiplexed channels operative at multiple gigabits per second.

This is a division of application Ser. No. 08/862,805, filed May 23,1997, is now patented as U.S. Pat. No. 5,831,761, which is, in turn, acontinuation of application Ser. No. 08/599,702, filed Feb. 9, 1996,issued as U.S. Pat. No. 5,719,696, which is, in turn, a division ofapplication Ser. No. 08/069,952, filed May 28, 1993, issued as U.S. Pat.No. 5,587,830 on Dec. 24, 1996.

BACKGROUND OF THE INVENTION

1. Technical Field

The field addressed concerns high capacity optical fiber networksoperative with wavelength division multiplexing. Systems contemplated:arm based on span distances which exceed 100 kilometers; depend uponsignal amplification rather than repeaters within spans, and use threeor more multiplexed channels each operative at a minimum of 5.0 gbitsper second.

2. Description of the Prior Art

The state of the art against which the present invention is consideredis summarized in the excellent article, "Dispersion-shifted Fiber",Lightwave, pp. 25-29, November 1992. As noted in that article, mostadvanced optical fiber systems now being installed and in the planningstages depend upon dispersion-shifted fiber (DS fiber). A number ofdevelopments have led to a preference for a carrier wavelength at 1.55μm. The loss minimum for the prevalent single-mode silica-based fiber isat this wavelength and the only practical fiber amplifier at thistime--the erbium amplifier operates best at this wavelength. It has beenknown for some time that the linear dispersion null point--the radiationwavelength at which the chromatic dispersion changes sign and passesthrough zero--naturally falls at about 1.31 μm for silica-based fiber.DS fiber--fiber in which the dispersion null point is shifted to 1.55μm--depends upon balancing the two major components of chromaticdispersion; material dispersion and waveguide dispersion. Waveguidedispersion is adjusted by tailoring the fiber's index-of-refractionprofile.

Use of DS fiber is expected to contribute to multi-channel operation--towavelength division multiplex (WDM). Here, multiple closely spacedcarrier wavelengths define individual channels, each operating at highcapacity--at 5.0 gbit/sec or higher. Installation intended for WDMeither initially or for contemplated upgrading uses three or morechannel operation, each operating sufficienty close to the zerodispersion point and each at the same capacity. Contemplated systems aregenerally based on four or eight WDM channels each operating at orupgradable to that capacity.

WDM systems use optical amplification rather than signal regenerationwhere possible. WDM becomes practical upon substitution of the opticalamplifier for the usual repeater which depends upon electronic detectionand optical regeneration. Use of the Er amplifier permits fiber spans ofhundreds of kilometers between repeaters or terminals. A system in theplanning stage uses optical amplifiers at 120 km spacing over a spanlength of 360 km.

The referenced article goes on to describe use of narrow spectral linewidths available from the distributed feedback (DFB) laser for highestcapacity long distance systems. The relatively inexpensive, readilyavailable Fabry Perot laser is sufficient for usual initial operation.As reported in that article, systems being installed by Telefonos deMexico; by MCI; and by AT&T are based on DS fiber.

A number of studies consider non-linear effects. (See, "Single-ChannelOperation in Very Long Nonlinear Fibers With Optical Amplifiers at ZeroDispersion" by D. Marcuse, J. Lightwave Technology, vol. 9, No. 3, pp.356-361, March 1991, and "Effect of Fiber Nonlinearity oil Long-DistanceTransmission" by D. Marcuse, A. R. Chraplyvy and R. W. Tkach, J.Lightwave Technology, vol. 9 No. 1, pp. 121-128, January 1991.)Non-linear effects studied include: Stimulated Brillouin Scattering;Self-Phase and Cross-Phase Modulation; Four-Photon Mixing (4 PM); andStimulated Raman Scattering. It has been known for some time thatcorrection of the linear dispersion problem is not the ultimatesolution. At least in principle, still more sophisticated systemsoperating over greater lengths and at higher capacities would eventuallyrequire consideration of non-linear effects as well.

TERMINOLOGY

WDM--Wavelength Division Multiplex, providing for multi-channeloperation within a single fiber. Channels are sufficiently close to besimultaneously amplified by a single optical amplifier. At this time,the prevalent optical amplifier (the erbium-doped silica fiberamplifier) has a usable bandwidth of Δλ≅10-20 nm.

Dispersion--When used alone, the term refers to chromatic dispersion--alinear effect due to wavelength dependent velocity within the carrierspectrum.

Span--Reference is made here to a repeaterless fiber length. This lengthwhich likely includes optical amplifiers is the distance betweenstations at which the signal has been converted from or is converted toelectronic form (commonly the distance between nearest signalregenerators). This span may define an entire system, or may be combinedwith one or more additional spans.

SUMMARY OF THE INVENTION

The claimed invention pertains to optical fiber suitable for use in WDMcommunication systems.

In most relevant terms, new installations for initial or contemplatedWDM optical fiber communications systems require fiber having a minimaldispersion over substantially the entirety of a communicationsspan--prohibit use of any substantial length of DS fiber. Spans may bemade up of uniform fiber of constant dispersion desirably at a value ofat least 1.5 ps/nm-kn. Alternatively, spans may use series of fiber ofdifferent dispersion by: "Concatenation" or "Compensation". Both includefiber of dispersion larger than 15 ps/nm-km. Concatenation usessuccessive fiber lengths of positive and negative dispersion generallyof the same order of magnitude. Compensation uses relatively shortlengths of "dispersion matching" fiber of very large dispersion tocompensate for major fiber lengths of opposite sign of dispersion. Whilenear-future WDM system use is tolerant of a small prescribed averageamount of chromatic dispersion, contemplated systems permit averaging toλ₀ =1550 nm. There is some preference for maintaining dispersion belowsome maximum value for any given length of fiber in the system.Particularly for systems of total capacity greater than 40 gbit/secfour-channel or 80 gbit/sec eight-channel, spontaneous generation toincrease spectral content beyond that introduced by thecarrier-generating laser, may result in capacity-limiting dispersion.Since resulting chromatic dispersion is effectively non-linear, theinitial pulse content is no longer retrievable. For these purposes, amaximum dispersion value of 8 ps/nm-km may be prescribed for moresophisticated systems of the future.

Enhanced signal capacity is due to fiber-path design which avoidsfour-photon mixing as the capacity limitation. This consideration isdetermining for: four or more channel systems with spacings of 2.5 nm orless; for span lengths at least equal to 300 km; permitting amplifierspacings of at least 100 km. The invention is defined accordingly.

U.S. Pat. No. 5,327,526, issued Jul. 5, 1994, claims a fiber of profileassuring a small but critical chromatic dispersion suitable for use inWDM systems. Its use is contemplated in a species of this invention.

In its broadest terms, the invention reflects the observation thatfour-photon mixing is a relevant mechanism which must be considered inthe design of contemplated WDM systems. A number of factors lendassuance to the assumption that the inventive teaching will take theform described above. For one thing, changing the carrier wavelength,e.g. to λ=1550±20 nm, for introducing requisite dispersion into DSfiber, while in principle appropriate, is not easily achievable. Theerbium amplifier at its present advanced state of development, has anoperating peak near 1550 nm. Operation 20 nm off this peak reduces thecarrier power level to an inconveniently low magnitude for one or moreof the channels. It is conceivable that substitution for erbium or thatsome other change in design of the amplifier will permit this operation.It is more likely that future systems will continue to be designedtaking advantage of the present or some more advanced stage of theconventional erbium amplifier.

Four-photon mixing depends upon the precise wavelengths of generatedcarriers. Evenly spaced, four-channel systems unavoidably satisfy thisrequirement.

The likely significance of 4 PM is somewhat reduced for a three-channelsystem, and precise uneven spacing even in a four-channel system may, inprinciple, avoid it as well. Problems in taking this approach requireoperating parameters which may be beyond the present state of the art,and, which in any event, would introduce some further expense. Reliablestabilization to maintain such precision, e.g. as due to thermal drift,is problematic.

These alternative approaches may not be seriously considered for newlyinstalled systems, but may be of value for upgrading of ingroundsystems--particularly, those with DS fiber in place.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a WDM system which serves fordiscussion of the various inventive species.

FIGS. 2-5 are "eye" diagrams which, as plotted on coordinates of powerand time, show the contrast between ones and zeros in the bit stream asdue to the various forms of dispersion including linear dispersion and 4PM for a four-channel system. The basic operating system characteristicsfor all of these figures are the same. They differ in thecharacteristics of the fiber.

DETAILED DESCRIPTION

General

It has now been found that the ultimate purpose to be served by DSF isthwarted by the very perfection with which chromatic dispersion iseliminated The permitted dispersion tolerance, of <3.5 ps/nm-km over thewavelength range λ=1525-1575 nm, of the DSF Specification Table is, initself, assurance of sufficient non-linearity to cause difficulty in WDMoperation, even in near-term systems. It is now found that plannedsystems arm incapable of operation due to a form of non-linearity. Thelimiting non-linearity--four-photon mixing (4 PM)--has been known forsome time and is described in the literature, see, article entitled"Effect of Fiber Nonlinearity on Long-Distance Transmission", citedabove. For most purposes 4 PM has been considered of academic interestonly. The cited paper is reasonably representative in examining systemsof span lengths of 7500 km. In-place systems (based on usual spanlengths, which are much shorter), as well as continued sale andinstallation of DSF specifically for WDM operation is consistent withthis view.

It is possible to lessen limitations imposed by 4 PM by sophisticatedcircuit design. Attention to channel spacings and modulation formats maypermit continued use of DSF for WDM systems of severely reducedcapability--for limited numbers of channels and for limited distances.WDM systems now contemplated, are not permitted, but become possible bypracticing the invention. Replacement of DSF will permit sought-forcapability of e.g., four-channel operation, per channel capacity of atleast 5 Gb/sec; repeaterless span lengths of 360 km and more, andchannel spacings of1.0 nm-2.0 nm. System designers will readilyacknowledge and implement the teaching.

As elsewhere in this description, specific magnitudes may beillustrative, or maybe be designed to satisfy near-term practical goals.As an example, channel spacings of 1.0 nm or greater take account ofreadily attainable frequency stabilization of transmitters andreceivers. Closer spacing with its greater permitted system capacity,taking advantage of the reduction in 4 PM of the invention, may bejustified. Design considerations have led to postulated spacings at 0.8nm.

The teaching depends on background knowledge of the skilled reader. Tobe rigorous, 4 PM appears as a fluctuating gain or loss--as a powerpenalty--due to constructive and destructive interference entailingsignals of different channels. 4 PM is not a noise source. Since theeffect is a signal distortion, with amplitude of some portions increasedand some decreased, the effect may not later be redressed. Since themagnitude of 4 PM is power dependent, the effect may be lessened byreducing launch power. For a given fiber span length, insertion loss maybe lessened, by the approach of increasing the number of amplifiers topermit a decrease in launched powers. As defined under "Terminology",WDMW permits use of amplifiers, each operating at a power levelprecluded by DSF for contemplated WDM. For these purposes, the inventiveadvance is defined in terms of amplifier spacings of 120 km or more withone or more amplifiers operating at a launch power level of 2.5mw/Gb-sec.

These considerations are in terms of an expected loss budget includingsplice losses, aging effects, etc., of 33 dB for the interamplifierspacing. Other considerations may suggest otherwise. As an explicitexample, undersea systems may use substantially greater span lengthsthan contemplated for terrestrial use due to greater installation andmaintenance costs of regenerator equipment This in turn leads to closeramplifier spacings--to spacings≦100 km.

Systems of the invention satisfy high level expectations of the systemdesigner--expectations now shown to be precluded with DSF.

FIG. 1 shows a characteristic WDM system as contemplated forinstallation in the near future. It consists of four transmitters, 10,11, 12, and 13, combined in a passive 4:1 coupler 14. The combinedsignal is introduced into fiber transmission line 15 which is providedwith two optical amplifiers 16 and 17. At the receiver end, thefour-channel signals are split by demultiplexer 18 after which theseparated signals are routed to the four regenerators 19, 20, 21 and 22.

FIG. 1 is representative of systems of the invention which may include agreater number of channels--8-channel systems are now contemplated.Longer systems may include longer spans or multiple spans so that thefour transmitters may serve for regeneration. For one system in theplanning stage, span length is 360 km and amplifier spacing is 120 km.Channel spacing, the difference in carrier wavelength is 200 GHz (orabout 1.5 nm). A fiber path may, as discussed, consist largely ofunchanging fixed dispersion fiber end-to-end, or may be made up ofconcatenated or compensated fiber.

WDM systems claimed differ from those presently planned primarily in thenature of the fiber transmission line. Previous systems were designed onthe premise that chromatic dispersion is the controlling factor oncapacity. It was expected that use of dispersion shifted fiber wouldpermit the WDM objective--initially span length of 360 km, four-channel,with per channel capacity of 5 gbit/sec. The thrust of the invention isthat a form of non-linear dispersion, four-photon mixing (4 PM),prevents attainment of the four-channel 20 gbit/sec capacity objective.The immediate result is to preclude use of any substantial length of DSfiber. It is expected that newly-installed systems will now usedispersive fiber. Any chromatic dispersion limit imposed will be offsetby concatenation or compensation.

The two approaches permit use of fiber having substantial values ofdispersion--permit use of fiber of dispersion greater than 4 ps/nm-kmand more as measured at λ=1550 nm. Both require precisely prescribedfiber lengths to exactly compensate and reduce dispersion to a suitablelevel The first, concatenation, uses successive lengths of "normal"dispersive fiber of opposite sign of dispersion. By "normal" is meantfibers of dispersion at or below that introduced by the materialdispersion of the system--for fiber now in use, at or below ˜18ps/nm-km. The approach is taken seriously for underwater installations,but has generally been disregarded for terrestrial use. It does requireprecise length determinations for each type of fiber beforeinstallation. The second, compensation, uses relatively short lengths ofhigh dispersion fiber, to compensate for the normal fiber. It isexpected that compensation fiber will be put on reels to be installed atamplifier or terminal points.

FIGS. 2-5

The "eye" diagrams of these figures trace channel power as a function oftime.

The diagrams are generated by plotting the received signal as a functionof time, and then shifting the time axis by one bit interval andplotting again. The abscissa interval is about 1 bit long. The 64now-superimposed bits define most probable (constructive anddestructive) interference events due to transmission in the threechannels adjoining the particular channel plotted. The eye diagramdepicts the worst case impairment as measured by the greatest ordinatevalue clear of traces (by the vertical dimension of the clear spacebetween a peak and null). A system which is not excessively impairedshows clear discrimination between "ones" and "zeros" with a large "eyeopening" in the center of the diagram. An unimpaired system isconsidered to have an "eye opening" of 1.0. Real systems which operateat openings of ˜0.9, are considered substantially unimpaired. Systemsare designed for such openings, so that substantially greater impairmentcalls for costly design modification--in the instance of WDM--bydecreasing amplifier/compensation distances and/or by reducing amplifierlaunch power.

Diagrams show a 64-bit pattern and include effects of both (linear)dispersion and those arising from non-linear index of refraction. Forconsistency, all curves are for the 3rd channel. Responsible factors areprimarily chromatic dispersion, 4 PM, and SPM. Operating power levelsare sufficiently low that other non-linear effects may be ignored.(Non-linear effects at a very low level are: Stimulated BrillouinScattering, and Stimulated Raman Scattering). Spurious lines areresponsive to all probable interactions. The significance of the diagramis in the "opening of the eye"--in the fraction uninhabited spacebetween a peak and a null.

FIG. 2 is the eye diagram for a DSF four-channel WDM system operatingwith: 200 GHz (1.5 nm) channel spacing; 360 km span length; 120 kmamplifier spacing; and operating at 5 Gb/sec per-channel capacity. Itsopening of ˜0.560 is inadequate for operation. Since non-dispersive,dispersion and SPM may be ignored so that eye closing is entirely due to4 PM.

FIG. 3 is the eye diagram for a WDMF system operating under the sameconditions. Its eye opening of ˜0.814 is sufficient contrast foroperation. The system of this figure is not compensated for itsdispersion of +2 ps/nm-km. Use of compensating fiber to reduce itsdispersion will further improve operation, which, although not neededunder these conditions, will permit increased capacity.

FIG. 4, again for the same WDM system, shows the use of fiber of adispersion of +16 ps/nm-km. The dispersion value is sufficienty highthat 4 PM under the operating conditions is insignificant. Spuriouslines are due to dispersion and SPM. The opening is ˜0.414.

FIG. 5 plots all factors of FIG. 4 but with compensation to null thedispersion at each amplifier position (with 120 km inter-amplifierspacing). Compensation based solely on the (linear) dispersion, whileignoring SPM entirely, increases the eye opening to ˜0.924. Based onthis plot, there is no reason to expect that SPM need be taken intoaccount, at least for compensation over the 120 km line lengths of thesystem, under the recited operating conditions.

SPM induced closure is a non-linear effect. Compensating over a greaterlength, e.g. by placement of compensation fiber only at termini of thespan, increases closure more than 3-fold due to this effect. The diagramsuggests that even this would be of little consequence. Preference forfiber of lesser dispersion--e.g.≦8 ps/nm-km--is expected to be ofconcern only for systems of substantially greatercompensation-to-compensation distances or of significantly greatercapacity.

I. The Transmission Line

A) WDM Fiber

DS fiber requires neither concatenation nor compensation and it islargely for this reason that it has been favored over the otherapproaches. The WDM fiber of U.S. patent application Ser. No.08/069,962, is now issued as U.S. Pat. No. 5,327,516, is expected toreplace DS fiber for near-term systems that are intolerant of dispersionnulling. This fiber, with chromatic dispersion within its permittedrange of 1.5-4 ps/nm-km, will likely be used for four-channel, 360 kmspan lengths, 20 gbit/sec systems. Future systems, of much highercapacity/span length, may use WDM fiber lines which are compensated tofurther reduce linear dispersion. For reasons described in the co-filedapplication, the sign of the dispersion required for WDM fiber, ispreferably positive (+1.5-4 ps/nm-km). Compensating fiber wouldaccordingly be of negative dispersion. As noted in the co-filedapplication, implications of the inventive teaching go beyond thedispersion range noted. Specification of this range is appropriate onbalance for contemplated systems. Use of lesser dispersion--to 1.0ps/nm-km and smaller--continues to ensure improved capacity over use ofDSF, although somewhat reduced as compared with the specified range.

While WDMF, as noted, may be used without equalization while satisfyingmany system requirements, equalization may further increase capacity. Inaddition to possible equalization by use of compensation fiber, aspecific form of concatenation is appealing. Here, concatenation wouldentail WDMF lengths of opposite sign of dispersion--both lengths withinthe preferred dispersion range of 1.5-4 ps/mn-km.

A trial specification table for WDM fiber suitable for use in anear-term system is set forth:

    ______________________________________                                        WDM specification Table                                                       ______________________________________                                        Attenuation attenuation                                                                     0.22-0.25 dB/km                                                 at 1550 nm                                                                    Attenuation at 1310 nm                                                                      0.45-0.50 dB/km                                                 Mode field diameter                                                                         8.4 ± 0.6 micron                                             Core eccentricity                                                                           Less than or equal to 0.8 micron                                Cladding diameter                                                                           125 ± 2.0 micron                                             Cut-off wavelength                                                                          <1.30 micron, (2 m reference length)                            Dispersion    ≧+2 ps/nm-km @ 1550 nm                                   Dispersion slope                                                                            <0.095 ps/nm.sup.2 -km maximum                                  Macrobending  <0.5 dB @ 1550 nm one turn, 32 mm                                             <0.1 dB @ 1550 nm 100 turns, 75 mm                              Coating diameter                                                                            250 ± 15 micron                                              Proof test    50 kpsi minimum (high proof test levels                                       available upon request)                                         Reel length   2.2, 4.4, 6.4, 8.8, 10.8, 12.6 and 19.2 km                      ______________________________________                                    

Design considerations are with a view to the small but criticaldispersion which is the primary differentiation over DSF. Other designcriteria regarding, inter alia, macrobending loss, mode field diameter,etc., are generally consistent with design of state-of-the art fiber(e.g. DSF) and may change as advances are made. AT&T Technical Journal,vol. 65, Issue 5, (1986) at pp. 105-121 is representative. Fiber issilica based, and includes a germania-doped core, together with one ormore cladding layers which may be of silica or may be down doped withfluorine. The overall 125 μm structure has a core of a diameter of about6 μm. The index peak has a Δn 0.013-0.015 with reference to undopedsilica. Usual profile is triangular or trapezoidal, possibly above a 20μm platform of Δn≅0.002. The WDM fiber specified may be compensated by aspool of compensating fiber. Compensating fiber of co-pending U.S.patent application Ser. No. 07/978,002, filed Nov. 18, 1993, is nowabandoned, is suitable for this purpose. Illustrative structures have adispersion of 2 ps/nm-km.

B) Compensation

The principle has been described. It is likely to take the form of amajor length of fiber of positive sign of dispersion, followed bycompensating fiber of negative dispersion. As with WDM fiber,compensating fiber may be of the form described in the co-pending U.S.patent application.

Self-Phase Modulation, a non-linear effect resulting in randomgeneration of different wavelengths, is found to be small. From FIGS. 4and 5, it is concluded that compensation for (linear) dispersion atappropriate distances (in that instance at 120 km spaced amplifierpositions) effectively eliminates SPM as a consideration. Under thesecircumstances, fiber with λ₀ =1310 nm is acceptable (disregarding costand inconvenience of compensation). The near-term WDM system on whichdescription is based (360 km span length, four-channel, 5 gbit/channel)does accept the ˜17 ps/nm-km uncorrected material dispersion of λ₀ =1310nm fiber. Future systems of longer spans or of greater capacity may usefiber of ˜8 ps/nm-km dispersion.

Consideration of SPM leads to compensation several ties along each spanlength. Requirements for the near-term WDM system are met bycompensation of the ˜17 ps/nm-km fiber at each amplifier (e.g. atspacings of 120 km). The inventive advance is useful for systems ofshorter span length as discussed Equalization (by compensation orconcatenation) should not be at such short lengths as to act as anoverall DS fiber. Equalization at distances of 1 km is precluded forthis reason. Lengths of less than 20 km are best avoided. Practicalsystem design, providing for tens of kilometers (e.g. 50 km or greater)of unequalized fiber for economic reasons, is suitable.

C) Concatenation

Considerations on system performance are quite similar to those forcompensation. Concatenation over fiber lengths much shorter than about20 km result in line behavior approaching that of DS fiber. Again,expedient design, with unequalized lengths of tens of kilometers isappropriate. SPM, an additional possibly limiting non-linear effect, canbe tolerated for contemplated 20 gbit four-channel systems. Plannedupgrading as well as higher capacity new installations may set apreferred maximum dispersion at ˜8 ps/nm-km.

As with compensation, concatenation offers complete elimination ofaverage dispersion. WDM systems presently planned may not require suchprecision. It is sufficient to reduce dispersion to that of the WDMFiber specification table set forth (≧2.0 ps/nm-km).

It is not expected that concatenation will play a major role in nearterm terrestrial systems. It is more likely in undersea systems.

D) Other Considerations

Span length has been discussed in terms of a contemplated system. There,provision is made for spans as great as 360 km. It is likely such asystem will contain shorter span lengths as well. This consideration maybe described in broader terms. The basic approach is useful for all WDMsystems, if only in permitting design freedom and relaxing designtolerances. A 5 gbit/sec, four-channel system gains significantly fromthe present teaching for span lengths of approximately 200 km. Therelationship between capacity and span length is defined by:

    B.sup.2 L≦104000/D                                  (Eq. 1)

where:

B=bit rate in gbit/sec

L=length in km

D=average dispersion in ps/nm-km

Since length varies as the square of the bit rate, the correspondingspan length for a 10 gbit/sec line capacity is 50 km. In general terms,then, systems based on the inventive teaching, include at least onefiber span in accordance with Eq. 1.

II. The Transmitter

This element as well as the receiver and optical amplifier are describedin detail in "Fiber Laser Sources and Amplifiers IV", SPIE, vol. 1789,pp. 260-266 (1992). The transmitter consists of a laser for eachchannel. Laser outputs are separately modulated and modulated signalsare multiplexed to be fed into the transmission line.

III. The Receiver

This element, at the end of a span length, may be at the system terminusor may be part of a signal regenerator. It includes a means fordemultiplexing the channels. This requires a device which passes thechannel wavelength of interest while blocking the others. This may be asimple splitter combined with optical fibers at the output ports tunedto each channel (SPIE ref. cited in preceding paragraph) or may be adevice which combines the functions of splitting and filtering in asingle unit.

IV. Optical Amplifier

This element, today, is an erbium amplifier. The useful gain region of asingle erbium amplifier is λ=40-50 nn. When amplifiers are connected ina series, the net gain narrows (since the amplitude within the "gainregion" is reduced on either side of the peak). The 10-20 nm bandwidthreferred to is a reasonable value for a three-amplifier span.

V. Other Considerations

For the most part, other considerations are standard With fewexceptions, WDM systems designed for use with DS Fiber may be directlyused for the invention. System design is in accordance withconsiderations common to the prior art and the invention. Channelspacing is necessarily such as to fit the channels within the peak ofthe optical amplifier. Span length maxima are set by insertion loss,launch power, and tolerable pulse spreading. Considerations may betailored naturally in accordance with constraints imposed For example,use of WDM fiber without compensation sets a limit on the product of bitrate and span length. Span length may be set by convenience, e.g. wherecompensation is to be provided, or where a concatenated fiber length isto begin.

Planned WDM systems use, external modulation to lessen dispersionpenalty and to improve the spectral stability of the channels.

We claim:
 1. Article comprising at least one optical fiber suitable foruse in wavelength division multiplex systems, the fiber including a coreand a clad, having an attenuation at 1550 nm≦0.25 dB/km and a dispersionslope <0.095 ps/(nm² -km),CHARACTERIZED IN THAT the absolute magnitudeof the average chromatic dispersion at 1550 nm for a fiber length of atleast 2.2 km is in the range of 1.5-8 ps/nm-km.
 2. Article of claim 1 inwhich the said average chromatic dispersion is at least 2 ps/nm-km.